Polyimides and particularly polyetherimides are known to be useful high performance polymers. Polyetherimides combine the high temperature characteristics of polyimides but still have sufficient melt processability to be easily formed by conventional molding techniques such as compression molding, gas assist molding, profile extrusion, thermoforming and injection molding. While the melt processability of polyetherimides has been very useful for forming parts, drawbacks have been seen regarding the build up of deposits on the equipment used to process the molten polymer and to form parts. This has been especially notable during the injection molding process. When molding, especially during prolonged molding operations, a deposit builds up on the processing equipment, for example in the mold itself. This build up is often referred to as plate out. Plate out can interfere with the proper operation of the melt processing equipment and can impair the performance and aesthetics of parts made by the process. As such these mold deposits must be periodically removed from the mold and equipment. This means shutting down the equipment and cleaning the mold, usually by hand. This exposes operators to potentially dangerous situations as well as causing a loss in production while the equipment and/or mold is being cleaned. Therefore, mold deposits accompanying the melt processing of polyetherimide resin present a problem.
In U.S. Pat. No. 4,910,288 Dellacoletta describes low molecular weight species in polyetherimide (PEI) resin arising from the use of a phthalic anhydride (PA) end capping agent and describes a process to reduce the low molecular weight species. In a phthalic anhydride capped polyetherimide made by reaction of bisphenol-A dianhydride with m-phenylenediamine (MPD) a low molecular weight species is formed by reaction of two moles of PA with one mole of MPD. This compound is phthalic anhydride meta-phenylene diamine imide (PAMI). PAMI is one of the main low molecular weight constituents of plate out found during molding of commercial polyetherimide products. While the use of the process of U.S. Pat. No. 4,910,288 reduces PAMI content, it is not eliminated, and PAMI is still present. In addition the manufacturing process taught in U.S. Pat. No. 4,910,288 is more complex and takes an inordinate period of time to complete. The longer process of U.S. Pat. No. 4,910,288 results in inefficient use of the very expensive equipment used to make the polyimide resin.